1. Field of the Invention
This invention relates to humidity sensing devices, and more particularly, to a humidity sensing structure having a cantilever resistor and a method for fabricating the same.
2. Description of Related Art
In recent years, temperature/humidity sensing devices are not only designed to be readable, but they also work with other portable electronic devices such as a cellular phone and applications in a smart car for comfortable and safe driving. As regards environmental safety monitoring, a temperature/humidity sensing component is required to be highly sensitive and high response speed. However, no temperature/humidity sensing component presently available in the market is good enough to satisfy these two requirements.
There are four types of temperature/humidity sensing components in the market, namely integrated electrode (IDE), piezoresistive type, surface acoustic wave (SAW), and optic temperature/humidity sensing component. They have their own advantages, but they have some disadvantages in common, such as low sensitivity, poor stability, poor linearity, low response speed, and that it is hard to compensate a drift value due to variance of temperature. In order to solve the drawbacks of the prior art and satisfy the demand for compact, portable sensing modules and their integration with a system, an integrated humidity sensor is introduced to the market.
FIG. 1 shows a schematic diagram of a micro-sensor disclosed by Taiwanese Publication No. 200508590 “MICRO-SENSOR AND METHOD FOR FABRICATING THE SAME AND SENSING DEVICE HAVING THE MICRO-SENSOR”. The micro-sensor comprises a first base 11 having a cantilever beam 110, a second base 12 opposite to the first base 11, a bridge board 13 installed on the first base 11 and the second base 12, a first electrode layer 14 formed on the bridge board 13, a humidity sensing layer 15 formed on the first electrode layer 14, a groove 130 formed on a bottom portion of the bridge 13, and a second electrode layer 16 formed in the groove 130 and corresponding in position to the first electrode layer 14. The first electrode layer 14 combines with the second electrode layer 16 to form a capacitor.
The humidity sensing layer 15 contracts or elongates, depending on humidity and the corresponding amount of water it absorbs. In consequence, the cantilever beam 110 bends to different degrees. A resultant change in the distance between the first electrode layer 14 and the second electrode layer 16 causes the capacitance of the capacitor to change. The aforesaid mechanism can be applied to the measurement of ambient humidity.
A method for fabricating the micro-sensor is described in FIGS. 2A to 2G. As shown in FIG. 2A, the method involves providing a substrate 10 having two mask layers 101 formed on a top surface and a bottom surface of the substrate 10. As shown in FIG. 2B, the method involves depositing the first electrode layer 14 on the mask layer 101 formed on the top surface of the substrate 10. As shown in FIG. 2C, the method involves forming the humidity sensing layer 15 on the first electrode layer 14. As shown in FIG. 2D, the method involves etching the mask layers 101 formed on the top and bottom surfaces of the substrate 10 to form two notches 102 and 103 respectively. As shown in FIG. 2E, through the notches 102 and 103, the substrate 10 is etched so as to form the first and second bases 11 and 12 and form the cantilever beam 110 on the first base 11. As shown in FIG. 2F, the method involves depositing metal in the groove 130 of the bridge board 13 to form the second electrode layer 16. As shown in FIG. 2G, the method involves connecting the groove 130 of the bridge board 13 to the first and second bases 11 and 12 such that the groove 130 faces downward and abuts against the first and second bases 11 and 12, to ensure that the second electrode layer 16 in the groove 130 is corresponding in position to the first electrode layer 14 on the cantilever beam 110, so as to form a capacitor required for the micro-sensor.
However, the micro-sensor is formed by stacking the first electrode layer 14 above the cantilever beam 110, which is movable, on the second electrode layer 16 below the bridge board 13, which is unmovable, the formation of the first and second electrode layers 14 and 16 and the stacking of the first electrode layer 14 on the second electrode layer 16 include three individual fabricating processes, and thus the micro-sensor of the prior art has a complicated fabricating process and a high fabricating cost.
Therefore, an existing issue which needs urgent resolution is about providing a micro-sensor having a simplified fabricating process and a low fabricating cost, but still having the original sensing characteristics.